U.S. patent application number 14/247349 was filed with the patent office on 2014-10-30 for physical quantity detection device and physical quantity detector.
This patent application is currently assigned to MITSUMI ELECTRIC CO., LTD.. The applicant listed for this patent is Yoshihiro Haseko, Taketomo Nakane, Yuki Shimazu. Invention is credited to Yoshihiro Haseko, Taketomo Nakane, Yuki Shimazu.
Application Number | 20140319628 14/247349 |
Document ID | / |
Family ID | 51767520 |
Filed Date | 2014-10-30 |
United States Patent
Application |
20140319628 |
Kind Code |
A1 |
Nakane; Taketomo ; et
al. |
October 30, 2014 |
PHYSICAL QUANTITY DETECTION DEVICE AND PHYSICAL QUANTITY
DETECTOR
Abstract
A physical quantity detection device includes a glass substrate,
a substrate including a physical quantity detection part and bonded
to a first surface of the glass substrate with a hermetically
sealed space being formed inside the substrate, and a function
membrane formed on a second surface of the glass substrate opposite
to the first surface. The function membrane prevents the second
surface of the glass substrate from coming into contact with
moisture in the atmosphere.
Inventors: |
Nakane; Taketomo; (Tokyo,
JP) ; Haseko; Yoshihiro; (Tokyo, JP) ;
Shimazu; Yuki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nakane; Taketomo
Haseko; Yoshihiro
Shimazu; Yuki |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
MITSUMI ELECTRIC CO., LTD.
Tokyo
JP
|
Family ID: |
51767520 |
Appl. No.: |
14/247349 |
Filed: |
April 8, 2014 |
Current U.S.
Class: |
257/415 |
Current CPC
Class: |
G01L 9/0042 20130101;
H01L 2924/00014 20130101; H01L 2224/48091 20130101; H01L 23/3121
20130101; H01L 29/84 20130101; H01L 23/10 20130101; G01L 9/0052
20130101; H01L 2224/73265 20130101; B81B 7/0025 20130101; H01L
23/564 20130101; H01L 2224/48091 20130101; B81B 2201/0264
20130101 |
Class at
Publication: |
257/415 |
International
Class: |
H01L 23/10 20060101
H01L023/10; H01L 29/82 20060101 H01L029/82 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2013 |
JP |
2013-092363 |
Claims
1. A physical quantity detection device, comprising: a glass
substrate; a substrate including a physical quantity detection
part, the substrate being bonded to a first surface of the glass
substrate with a hermetically sealed space being formed inside the
substrate; and a function membrane formed on a second surface of
the glass substrate opposite to the first surface, the function
membrane preventing the second surface of the glass substrate from
coming into contact with moisture in an atmosphere.
2. The physical quantity detection device as claimed in claim 1,
wherein the function membrane has a disposition to repel moisture
so as to prevent alkali metal ions in the glass substrate from
coming into contact with the moisture in the atmosphere and a
disposition to prevent the alkali metal ions from migrating through
the glass substrate.
3. The physical quantity detection device as claimed in claim 1,
wherein the function membrane is formed of a film comprising one of
gold, titanium and a silicon nitride.
4. The physical quantity detection device as claimed in claim 1,
wherein a thickness of the glass substrate is less than or equal to
a thickness of the substrate.
5. The physical quantity detection device as claimed in claim 1,
wherein a thickness of the glass substrate is less than or equal to
800 .mu.m.
6. The physical quantity detection device as claimed in claim 1,
wherein the substrate is formed of silicon.
7. A physical quantity detector, comprising: the physical quantity
detection device as set forth in claim 1.
8. The physical quantity detector as claimed in claim 7, wherein
the physical quantity detection device is fixed onto another
substrate by adhesive resin with the function membrane facing
toward said another substrate.
9. A physical quantity detection device, comprising: a first glass
substrate and a second glass substrate, wherein at least one of the
first and second glass substrates includes a cavity; a substrate
including a physical quantity detection part, the substrate being
bonded to and provided between the first and second glass
substrates with a hermetically sealed space formed around the
physical quantity detection part; and a first function membrane and
a second function membrane formed on a surface of the first glass
substrate facing away from the substrate and a surface of the
second glass substrate facing away from the substrate,
respectively, wherein the first and second function membranes
preventing the surfaces of the first and second glass substrates
from coming into contact with moisture in an atmosphere.
10. The physical quantity detection device as claimed in claim 9,
wherein the first and second function membranes have a disposition
to repel moisture so as to prevent alkali metal ions in the first
and second glass substrates from coming into contact with the
moisture in the atmosphere and a disposition to prevent the alkali
metal ions from migrating through the first and second glass
substrates.
11. The physical quantity detection device as claimed in claim 9,
wherein each of the first and second function membranes is formed
of a film comprising one of gold, titanium and a silicon
nitride.
12. The physical quantity detection device as claimed in claim 9,
wherein a thickness of each of the first and second glass
substrates is less than or equal to a thickness of the
substrate.
13. The physical quantity detection device as claimed in claim 9,
wherein a thickness of each of the first and second glass
substrates is less than or equal to 800 .mu.m.
14. The physical quantity detection device as claimed in claim 9,
wherein the substrate is formed of silicon.
15. A physical quantity detector, comprising: the physical quantity
detection device as set forth in claim 9.
16. The physical quantity detector as claimed in claim 15, wherein
the physical quantity detection device is fixed onto another
substrate by adhesive resin with one of the first and second
function membranes facing toward said another substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority of Japanese Patent Application No. 2013-092363, filed on
Apr. 25, 2013, the entire contents of which are incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to physical quantity detection
devices and physical quantity detectors.
[0004] 2. Description of the Related Art
[0005] Gauge pressure sensors having a silicon substrate including
a diaphragm joined to an upper surface of a glass substrate have
been known. In such gauge pressure sensors, a through hole for
communicating the pressure of a medium to be measured is provided
in the glass substrate. Furthermore, a metal film for soldering a
gauge pressure sensor to a metal base is formed on a lower surface
(bottom surface) of the glass substrate. It has been shown that the
metal film for soldering preferably has a layered structure of
different kinds of films, which is less likely to be degraded by
sodium ions included in the glass substrate. (See, for example,
Japanese Laid-Open Patent Applications No. 2000-241274 and No.
2-272339 and Japanese Laid-Open Examined Patent Application No.
6-76938.)
[0006] On the other hand, apart from the above-described gauge
pressure sensors, absolute pressure sensors are known that use a
physical quantity detection device in which a space hermetically
sealed by a glass substrate and a silicon substrate including a
diaphragm is formed by joining the silicon substrate to an upper
surface of the glass substrate. For example, a Wheatstone bridge
circuit using four piezoresistive elements whose resistance values
change depending on an applied pressure is formed on the
diaphragm.
[0007] In such absolute pressure sensors, a lower surface (bottom
surface) of the glass substrate of the physical quantity detection
device is fixed onto a substrate by an adhesive agent. The
thickness of the silicon substrate is, for example, approximately 1
mm, the thickness of the glass substrate is, for example,
approximately 1 mm, and the total thickness of the physical
quantity detection device is, for example, approximately 2 mm.
SUMMARY OF THE INVENTION
[0008] According to an aspect of the present invention, a physical
quantity detection device includes a glass substrate, a substrate
including a physical quantity detection part and bonded to a first
surface of the glass substrate with a hermetically sealed space
being formed inside the substrate, and a function membrane formed
on a second surface of the glass substrate opposite to the first
surface. The function membrane prevents the second surface of the
glass substrate from coming into contact with moisture in the
atmosphere.
[0009] According to an aspect of the present invention, a physical
quantity detection device includes a first glass substrate and a
second glass substrate, wherein at least one of the first and
second glass substrates includes a cavity, a substrate including a
physical quantity detection part, the substrate being bonded to and
provided between the first and second glass substrates with a
hermetically sealed space formed around the physical quantity
detection part, and a first function membrane and a second function
membrane formed on a surface of the first glass substrate facing
away from the substrate and a surface of the second glass substrate
facing away from the substrate, respectively, wherein the first and
second function membranes preventing the surfaces of the first and
second glass substrates from coming into contact with moisture in
an atmosphere.
[0010] According to an aspect of the present invention, a physical
quantity detector includes any of the physical quantity detection
devices as set forth above.
[0011] The object and advantages of the invention will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0012] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and not restrictive of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other objects, features and advantages of the present
invention will become more apparent from the following detailed
description when read in conjunction with the accompanying
drawings, in which:
[0014] FIG. 1 is a cross-sectional view of a physical quantity
detection device according to a first embodiment;
[0015] FIG. 2 is a plan view of a diaphragm surface of a physical
quantity detection device according to the first embodiment;
[0016] FIGS. 3A, 3B and 3C are diagrams illustrating a process for
manufacturing a physical quantity detection device according to the
first embodiment;
[0017] FIGS. 4A and 4B are graphs illustrating the dependence of
the output variation of a physical quantity detection device on the
thickness of a glass substrate;
[0018] FIG. 5 is another graph illustrating the dependence of the
output variation of a physical quantity detection device on the
thickness of a glass substrate;
[0019] FIG. 6 is a graph illustrating the dependence of the output
voltage of a physical quantity detection device on a bond
voltage;
[0020] FIG. 7 is a graph illustrating the dependence of the output
voltage of a physical quantity detection device on a bond
temperature;
[0021] FIG. 8 is a graph illustrating the dependence of the output
variation of a physical quantity detection device on the film type
of a function membrane;
[0022] FIG. 9 is a cross-sectional view of a physical quantity
detection device according to a first variation of the first
embodiment;
[0023] FIG. 10 is a cross-sectional view of a physical quantity
detection device according to a second variation of the first
embodiment;
[0024] FIGS. 11A and 11B are diagrams illustrating a physical
quantity detection device according to a third variation of the
first embodiment; and
[0025] FIG. 12 is a cross-sectional view of a physical quantity
detector according to a second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Consideration has been given to providing such absolute
pressure sensors as described above in small apparatuses such as
cellular phones. Therefore, there has been a demand for small and
low-profile absolute pressure sensors. Accordingly, absolute
pressure sensors are desired to be significantly reduced in the
thickness of the silicon substrate and the thickness of the glass
substrate.
[0027] The inventors of the present invention, however, have found
that when the thickness of the glass substrate is less than a
predetermined value, a variation in the output of the physical
quantity detection device (a variation in the output of the
Wheatstone bridge circuit) under predetermined conditions increases
as the thickness of the glass substrate decreases.
[0028] According to an aspect of the present invention, a physical
quantity detection device that reduces output variations
irrespective of the thickness of a glass substrate is provided.
[0029] A description is given, with reference to the accompanying
drawings, of embodiments of the present invention. In the drawings,
the same element is referred to by the same reference numeral, and
a description of the same element may not be repeated.
First Embodiment
[0030] FIG. 1 is a cross-sectional view of a physical quantity
detection device according to a first embodiment. Referring to FIG.
1, a physical quantity detection device 10 according to the first
embodiment includes a diaphragm part 20, a glass substrate 30, and
a function membrane 40. The physical quantity detection device 10
may be provided in, for example, absolute pressure sensors.
[0031] The diaphragm part 20 (a pressure-sensitive diaphragm part)
form a sensor surface of the physical quantity detection device 10.
The diaphragm part 20 detects a stress generated by a pressure by
converting the stress into an electrical signal. The diaphragm part
20 includes a diaphragm surface 21 and a diaphragm support part 22.
The diaphragm part 20 is an example of a physical quantity
detection part, and the physical quantity may be other than a
pressure.
[0032] The diaphragm surface 21 includes a surface where a pressure
is detected. The diaphragm surface 21 is formed into a thin film
shape. The diaphragm surface 21 is configured to detect a pressure
applied to the diaphragm surface 21 by the deflection of the
diaphragm surface 21 caused by the applied pressure. Furthermore,
the diaphragm support part 22 supports the diaphragm surface
21.
[0033] The diaphragm part 20 is formed in, for example, a silicon
(Si) substrate. In the following description, the silicon substrate
refers to a substrate whose principal component is silicon.
Examples of the silicon substrate include a silicon-on-insulator
(SOI) substrate. In the case of using an SOI substrate as the
diaphragm part 20, the diaphragm surface 21 may be formed with a
silicon active layer and the diaphragm support part 22 may be
formed with a buried oxide and a silicon substrate on the bottom
side.
[0034] The glass substrate 30 is a support member that supports the
diaphragm part 20. For example, a bottom surface of the diaphragm
support part 22 of the diaphragm part 20 is fixed to an outer edge
portion of a first surface 30a (a top surface in FIG. 1) of the
glass substrate 30 by anodic bonding. A multilayer glass substrate
may be used as the glass substrate 30.
[0035] The diaphragm part 20 is bonded to the first surface 30a of
the glass substrate 30, so that a cavity part 23, which is a
hermetically sealed space, is formed. In the case of providing the
physical quantity detection device 10 in an absolute pressure
sensor, the cavity part 23 serves as a vacuum reference chamber
maintained in a vacuum state.
[0036] The function membrane 40 is formed so as to cover a second
surface 30b (a bottom surface on the side opposite to the first
surface 30b in FIG. 1) of the glass substrate 30. In general, the
term "function membrane" refers to a thin film having a
predetermined function. In embodiments of the present invention,
the predetermined function is a function to prevent the second
surface 30b of the glass substrate 30 from coming into contact with
moisture in the atmosphere. That is, the function membrane 40
prevents alkali metal ions (such as Na.sup.+ and K.sup.+) included
in the glass substrate 30 from coming into contact with moisture in
the atmosphere. The material of the function membrane 40 may be
selected from those having a disposition to repel moisture and also
a disposition to prevent the migration of alkali metal ions (such
as Na.sup.+ and K.sup.+).
[0037] Examples of the function membrane 40 having these
dispositions include inorganic films such as metal films, silicon
nitride films (SiN films), and diamond-like carbon (DLC) films and
organic films of special polyurethane, fluorocarbon polymers,
acrylonitrile butadiene styrene (ABS) polymers, and
polystyrene.
[0038] Examples of metal films usable as the function membrane 40
include a titanium (Ti) film of approximately 100 nm in thickness
and a gold (Au) film of approximately 150 nm in thickness. The Ti
film and the Au film may be deposited by, for example, sputtering.
In the case of depositing a Au film, for example, a titanium
tungsten (TiW) film of approximately 35 nm in thickness may be used
as an underlayer. The specific meaning of forming the function
membrane 40 is described below.
[0039] FIG. 2 is a plan view of a diaphragm surface of a physical
quantity detection device according to the first embodiment.
Referring to FIG. 2, the diaphragm surface 21 includes
piezoresistive elements 211, impurity resistance interconnects 212,
metal interconnects 213, and pads 214. The piezoresistive elements
211 and the impurity resistance interconnects 212 form a Wheatstone
bridge circuit and are configured to detect output voltage.
[0040] The piezoresistive elements 211 are one type of
piezoelectric element, whose resistance values change depending on
an applied pressure. Therefore, the Wheatstone bridge circuit using
the piezoresistive elements 211 is configured to detect a pressure
applied onto the diaphragm surface 21 by a variation in the output
voltage. That is, a pressure applied to the diaphragm surface 21 is
detected by a variation in the output voltage commensurate with a
change in the resistance values of the piezoresistive elements
211.
[0041] Furthermore, the metal interconnects 213 are interconnects
for forming the Wheatstone bridge circuit. The pads 214 are
terminals or electrodes for external electrical connections. Power
is externally supplied to the pads 214 to apply voltage to the
Wheatstone bridge circuit, and a variation in the output voltage of
the Wheatstone bridge circuit is detected from a change in the
resistance values of the piezoresistive elements 211 due to the
application of a pressure. Thus, a pressure applied to the
diaphragm surface 21 is detected. For example, the physical
quantity detection device 10 detects a pressure by having the
diaphragm surface 21 configured as illustrated in FIG. 2.
[0042] The physical quantity detection device 10 may be
manufactured in the following manner, for example. In FIGS. 3A, 3B
and 3C, a single physical quantity detection device is illustrated.
In practice, however, multiple physical quantity detection devices
are formed on a single wafer and are finally separated into
individual physical quantity detection devices by dicing.
[0043] FIGS. 3A through 3C are diagrams illustrating a process for
manufacturing a physical quantity detection device according to the
first embodiment. First, at the process step illustrated in FIG.
3A, the diaphragm part 20 is formed. To be specific, for example, a
substrate including silicon as a principal component, such as a
silicon (Si) substrate or a SOI substrate, is prepared, and the
diaphragm part 20 of a predetermined shape is formed by performing
processing such as etching on the prepared substrate. A thickness
T.sub.1 of the diaphragm part 20 may be, for example, approximately
150 .mu.m.
[0044] Next, at the process step illustrated in FIG. 3B, the
diaphragm part 20 is fixed to the first surface 30a of the glass
substrate 30 by anodic bonding. To be specific, the diaphragm part
20 and the glass substrate 30 are brought into contact in a
high-temperature environment at a temperature (bond temperature)
of, for example, approximately 300.degree. C. to approximately
400.degree. C. Then, a high voltage (bond voltage) of, for example,
approximately 500 V to approximately 1500 V is applied across the
diaphragm part 20 and the glass substrate 30 from a direct-current
power supply with the diaphragm part 20 being at a higher potential
than the glass substrate 30. As a result, the diaphragm part 20 is
fixed to the first surface 30a of the glass substrate 30 by anodic
bonding, so that the cavity part 23, which is a hermetically sealed
space, is formed. A thickness T.sub.2 of the glass substrate 30 may
be, for example, approximately 100 .mu.m.
[0045] Next, at the process step illustrated in FIG. 3C, the
function membrane 40 is formed on the second surface 30b of the
glass substrate 30. The function membrane 40 may be formed by
depositing a Ti film on the second surface 30b of the glass
substrate 30 by sputtering, for example. The thickness of the
function membrane 40 may be, for example, approximately 100 nm. In
place of the Ti film, any of the films described above as examples
of the function membrane 40 may be formed. After the process step
illustrated in FIG. 3C, individual physical quantity detection
devices separated by dicing are formed.
[0046] The process step of forming the function membrane 40 is not
limited in particular. For example, physical quantity detection
devices may be separated by dicing before formation of the function
membrane 40, and thereafter, the function membrane 40 may be formed
on the second surface 30b of the glass substrate 30 of each
individual physical quantity detection device. Alternatively, the
function membrane 40 may be formed on the second surface 30b of the
glass substrate 30 before anodic bonding at the process step
illustrated in FIG. 3B. In this case, an area to be used as an
electrode part in anodic bonding is exposed in part of the second
surface 30b of the glass substrate 30.
[0047] Here, a description is given, with reference to the results
of the experiments conducted by the inventors, of effects of
forming the function membrane 40 on the second surface 30b of the
glass substrate 30.
[Dependence of Output Variation on Thickness of Glass
Substrate]
[0048] First, in the manner of the process steps illustrated in
FIGS. 3A and 3B, multiple physical quantity detection devices in
which the function membrane 40 was not formed on the second surface
30b of the glass substrate 30 were manufactured using the thickness
of the glass substrate 30 as a parameter. To be specific, five
physical quantity detection devices were manufactured with the
thickness T.sub.1 of the diaphragm part 20 being 1000 .mu.m, the
thickness T.sub.2 of the glass substrate 30 being 1000 .mu.m, and
the bond temperature and bond voltage of anodic bonding being
400.degree. C. and 600 V, respectively. Furthermore, apart from
these physical quantity detection devices, nine physical quantity
detection devices were manufactured with the thickness T.sub.1 of
the diaphragm part 20 being 150 .mu.m, the thickness T.sub.2 of the
glass substrate 30 being 100 .mu.m, and the bond temperature and
bond voltage of anodic bonding being 400.degree. C. and 600 V,
respectively.
[0049] Next, after leaving each physical quantity detection device
in a high-temperature, high-humidity condition (in an atmospheric
environment of 85.degree. C. and 85% Rh) for 100 hours, the
temperature was returned to normal temperature and the output
variation of each physical quantity detection device was measured
by feeding an electric current to each physical quantity detection
device. The measured output variation (hereinafter simply referred
to as "post-test output variation") is a value obtained by
converting a variation in the output voltage of the above-described
Wheatstone bride circuit between before and after the test into a
pressure (unit: Pa). The output voltage of the Wheatstone bride
circuit before the test is a reference (zero).
[0050] According to the inventors, the condition for acceptable
products is that the post-test output variation is within 100 Pa.
This is because it is empirically known that the output variation
of a physical quantity detection device falls within 150 Pa even
after leaving the physical quantity detection device in a
high-temperature, high-humidity condition (in an atmospheric
environment of 85.degree. C. and 85% Rh) for 1000 hours as long as
this condition is satisfied and because no problem is caused in
practical usage if the output variation is within 150 Pa. This
condition, however, is a mere example, and may be changed to
another condition depending on the use and/or design of the
physical quantity detection device.
[0051] FIGS. 4A and 4B are graphs illustrating the dependence of
the output variation of a physical quantity detection device on the
thickness of a glass substrate. FIG. 4A illustrates the measurement
results of the post-test output variation in the case where
T.sub.1=1000 .mu.m and T.sub.2=1000 .mu.m. FIG. 4B illustrates the
measurement results of the post-test output variation in the case
where T.sub.1=150 .mu.m and T.sub.2=100 .mu.m.
[0052] It has been confirmed from FIG. 4A that with respect to the
relatively thick physical quantity detection devices of
T.sub.1=1000 .mu.m and T.sub.2=1000 .mu.m, the post-test output
variation is within 100 Pa and the above-described condition for
acceptable products is satisfied. On the other hand, it has been
confirmed from FIG. 4B that with respect to the relatively thin
physical quantity detection devices of T.sub.1=150 .mu.m and
T.sub.2=100 .mu.m, the post-test output variation exceeds 100 Pa
and the above-described condition for acceptable products is not
satisfied.
[0053] Next, three physical quantity detection devices were
manufactured with the thickness T.sub.1 of the diaphragm part 20
being 150 .mu.m, the thickness T.sub.2 of the glass substrate 30
serving as a parameter, and the bond temperature and bond voltage
of anodic bonding being 400.degree. C. and 600 V, respectively, and
the post-test output voltage was measured in the same manner as
described above.
[0054] FIG. 5 is another graph illustrating the dependence of the
output variation of a physical quantity detection device on the
thickness of a glass substrate. It has been found from FIG. 5 that
with respect to the relatively thin physical quantity detection
devices of T.sub.1=150 .mu.m, the post-test output variation
increases as the thickness T.sub.2 of the glass substrate 30
decreases and the post-test output variation decreases as the
thickness T.sub.2 of the glass substrate 30 increases. Furthermore,
it has been found from FIG. 5 that with respect to the relatively
thin physical quantity detection devices of T.sub.1=150 .mu.m, the
thickness T.sub.2 of the glass substrate 30 needs to be more than
800 .mu.m in order to satisfy the above-described condition for
acceptable products.
[Dependence of Output Variation on Anodic Bonding Conditions]
[0055] At the above-described process step illustrated in FIG. 3B,
multiple physical quantity detection devices where the function
membrane 40 was not formed on the second surface 30b of the glass
substrate 30 were formed using the bond temperature and bond
voltage of anodic bonding as parameters. To be specific, multiple
physical quantity detection devices were manufactured with the
thickness T.sub.1 of the diaphragm part 20 being 150 .mu.m, the
thickness T.sub.2 of the glass substrate 30 being 100 .mu.m, and
the bond temperature and bond voltage of anodic bonding serving as
parameters, and the post-test output voltage was measured in the
same manner as described above.
[0056] FIG. 6 is a graph illustrating the dependence of the output
voltage of a physical quantity detection device on a bond voltage.
The three physical quantity detection devices illustrated in FIG. 6
were manufactured with the bond temperature of anodic boding being
400.degree. C. and the bond voltage of anodic bonding being 600 V,
1000 V, and 1500 V, respectively. It has been found from FIG. 6
that the relatively thin physical quantity detection devices of the
thickness T.sub.1 of the diaphragm part 20 being 150 .mu.m and the
thickness T.sub.2 of the glass substrate 30 being 100 .mu.m cannot
satisfy the above-described condition for acceptable products even
when the bond temperature of anodic bonding is controlled.
[0057] FIG. 7 is a graph illustrating the dependence of the output
voltage of a physical quantity detection device on a bond
temperature. The three physical quantity detection devices
illustrated in FIG. 7 were manufactured with the bond voltage of
anodic boding being 600 V and the bond temperature of anodic
bonding being 300.degree. C., 350.degree. C., and 400.degree. C.,
respectively. It has been found from FIG. 7 that the relatively
thin physical quantity detection devices of the thickness T.sub.1
of the diaphragm part 20 being 150 .mu.m and the thickness T.sub.2
of the glass substrate 30 being 100 .mu.m cannot satisfy the
above-described condition for acceptable products even when the
bond voltage of anodic bonding is controlled.
[Study of Function Membrane]
[0058] Next, multiple physical quantity detection devices having
the function membrane 40 formed on the second surface 30b of the
glass substrate 30 were manufactured in the manner of the process
steps illustrated in FIGS. 3A through 3C. To be specific, the
diaphragm part 20 and the glass substrate 30 were bonded by anodic
bonding with the thickness T.sub.1 of the diaphragm part 20 being
150 .mu.m, the thickness T.sub.2 of the glass substrate 30 being
100 .mu.m, and the bond temperature and bond voltage of anodic
bonding being 400.degree. C. and 600 V, respectively. Then, four
physical quantity detection devices were manufactured by depositing
the function membrane 40 on the second surface 30b of the glass
substrate 30.
[0059] In the four physical quantity detection devices, a Ti film
of 100 nm in thickness, a Au film of 150 nm in thickness, a SiN
film of 100 nm in thickness, and a silicon oxide (SiO.sub.2) film
of 100 nm in thickness were deposited, respectively, as the
function membrane 40. In the formation of the Au film, a TiW film
of 35 nm in thickness was deposited as an underlayer.
[0060] The Ti film, the Au film, and the underlayer TiW film were
deposited by sputtering. Furthermore, the SiN film and the
SiO.sub.2 film were deposited by plasma chemical vapor deposition
(CVD).
[0061] FIG. 8 is a graph illustrating the dependence of the output
variation of a physical quantity detection device on the film type
of a function membrane. FIG. 8 also illustrates data on a physical
quantity detection device having no function membrane (indicated by
"NONE") for comparison purposes. It has been found from FIG. 8 that
the relatively thin physical quantity detection devices of the
thickness T.sub.1 of the diaphragm part 20 being 150 .mu.m and the
thickness T.sub.2 of the glass substrate 30 being 100 .mu.m can
satisfy the above-described condition for acceptable products by
selecting a proper film type and depositing the function membrane
40 on the second surface 30b of the glass substrate 30.
[Summary]
[0062] The above-described experimental results are summarized as
follows. When the thickness T.sub.2 of the glass substrate 30 is
more than 800 .mu.m, it is possible to satisfy the above-described
condition for acceptable products irrespective of the presence or
absence of a function membrane. (See FIG. 5.) However, when the
thickness T.sub.2 of the glass substrate 30 is reduced (to 800
.mu.m or less) in order to satisfy a commercial demand for thinner
physical quantity detection devices, it is not possible to satisfy
the above-described condition for acceptable products. (See FIG.
5.) Furthermore, this result is not improved by changing anodic
bonding conditions (a bond temperature and a bond voltage). (See
FIG. 6 and FIG. 7.)
[0063] On the other hand, when the function membrane 40 is formed
on the second surface 30b of the glass substrate 30, it is possible
to significantly reduce the output variation compared with the case
of not forming the function membrane 40 and to satisfy the
above-described condition for acceptable products, depending on the
film type of the function membrane 40. (See FIG. 8.) That is, the
film type of the function membrane 40 is selected from those having
a disposition to repel moisture and also a disposition to prevent
the migration of alkali metal ions (such as Na.sup.+ and K.sup.+).
As a result, even when the thickness T.sub.2 of the glass substrate
30 is reduced (to 800 .mu.m or less), it is possible to
significantly reduce the output variation and thus to satisfy the
above-described condition for acceptable products.
[0064] Here, an explanation is given of the reason the formation of
the function membrane 40 on the second surface 30b of the glass
substrate 30 makes it possible to significantly reduce the output
variation even when the thickness T.sub.2 of the glass substrate 30
is reduced (to 800 .mu.m or less).
[0065] In anodic bonding, alkali glass is commonly used in
principle. The components of alkali glass include alkali metals
such as Na and K. At a surface of the glass, the following reaction
occurs between alkali metal ions such as sodium ions (Na.sup.+) and
potassium ions (K.sup.+) in the glass and H.sub.2O in the
atmosphere. The reaction illustrated below is about sodium ions
(Na.sup.+), but the same reaction also occurs with respect to
potassium ions (K.sup.+).
Na.sup.+ (glass)+H.sub.2O (in the atmosphere).fwdarw.NaOH+H.sup.+
(into glass). (1)
[0066] This reaction is more likely to occur at higher humidity and
higher temperature. Furthermore, silicates, which are principal
components of glass, hardly dissolve in acids (except a
hydrofluoric acid). Silicates, however, have poor resistance to
alkalis, and dissolve in alkaline solutions having a pH greater
than or equal to 9.8. Therefore, it is believed that generated NaOH
further captures moisture in the atmosphere to become an alkaline
solution so that glass is dissolved to be apparently reduced in
thickness, thus disrupting stress balance to cause a characteristic
variation.
[0067] In the case of a large glass thickness (for example, 1000
.mu.m), it is understood that the characteristic variation was
limited as illustrated in FIG. 4A because a layer of dissolved
glass is relatively limited and the stress balance of the physical
quantity detection device hardly changes. On the other hand, in the
case of a small glass thickness (for example, 100 .mu.m), it is
understood that the characteristic variation was conspicuous as
illustrated in FIG. 4B because a layer of dissolved glass is
relatively large and the stress balance of the physical quantity
detection device changes.
[0068] In anodic bonding, it is known that whitish powder (deposit)
whose component is Na adheres to a bottom surface of glass in
principle, and this component may become an alkaline solution.
Therefore, the inventors evaluated samples having glass polished
after anodic bonding in the same manner, but found no improvement.
From this result, it is clear that the output variation is not
caused by a deposit and it is understood that the output variation
is caused by the reaction at the glass surface as described
above.
[0069] Furthermore, depending on the film type of the function
membrane 40, formation of the function membrane 40 does not reduce
the output variation. To be specific, the output variation is not
reduced by forming a SiO.sub.2 film as the function membrane 40 as
illustrated in FIG. 8. The reason the formation of a SiO.sub.2 film
as the function membrane 40 had no effect is that alkali metal ions
such as N.sup.+ and K.sup.+ are allowed to move inside SiO.sub.2,
which is also a principal component of alkali glass.
[0070] That is, alkali metal ions such as N.sup.+ and K.sup.+ are
mobile ions and migrate to minimize the energy state inside glass.
Therefore, it is understood that alkali metal ions such as N.sup.+
and K.sup.+ migrate through the SiO.sub.2 film to react H.sub.2O in
the atmosphere, thus making the formation of the SiO.sub.2 film
ineffective.
[0071] Thus, according to the first embodiment, in the physical
quantity detection device 10 including the diaphragm part 20 and
the glass substrate 30, the function membrane 40 that prevents
alkali metal ions included in the glass substrate 30 from coming
into contact with moisture in the atmosphere is formed on the
second surface 30b of the glass substrate 30. As a result, even
when the glass substrate 30 is reduced in thickness (to 800 .mu.m
or less), the function membrane 40 prevents alkali metal ions in
the glass substrate 30 and moisture in the atmosphere from reacting
and generating an alkaline solution to dissolve the second surface
30b of the glass substrate 30. Consequently, it is possible to
satisfy a predetermined specification (the above-described
condition for acceptable products) with respect to the output
variation of the physical quantity detection device 10.
[0072] In the case of reducing the thickness of the physical
quantity detection device 10, while neither the thickness of a
silicon substrate nor the thickness of a glass substrate is
preferred in their size relationship, it is difficult to make the
silicon substrate thinner than 150 .mu.m. Accordingly, in the case
of particularly reducing the thickness of the physical quantity
detection device 10, the thickness of the glass substrate may be
made smaller than or equal to the thickness of the glass substrate.
In such a case, formation of the function membrane 40 is
particularly effective as a measure to reduce the output variation
of the physical quantity detection device 10. This, however, does
not limit the invention to making the thickness of the glass
substrate smaller than or equal to the thickness of the silicon
substrate.
[0073] In the case of providing the physical quantity detection
device 10 in absolute pressure sensors, the cavity part 23 is
maintained in a vacuum state and does not come into contact with
the atmosphere. Accordingly, there is no need to form a function
membrane on the first surface 30a of the glass substrate 30.
[First Variation of First Embodiment]
[0074] In a first variation of the first embodiment, a physical
quantity detection device having a structure different from that of
the first embodiment is illustrated. In the first variation of the
first embodiment, a description of the same elements as those of
the embodiment described above is omitted.
[0075] FIG. 9 is a cross-sectional view of a physical quantity
detection device according to the first variation of the first
embodiment. Referring to FIG. 9, a physical quantity detection
device 10A according to the first variation of the first embodiment
includes a diaphragm part 20A, a glass substrate 30A, and the
function membrane 40. The physical quantity detection device 10A
may be provided in, for example, absolute pressure sensors.
[0076] Unlike the diaphragm part 20 (FIG. 1) of the physical
quantity detection device 10, the diaphragm part 20A (a
pressure-sensitive diaphragm part), which is a physical quantity
detection part, has a flat plate shape. The diaphragm part 20A
operates the same as the diaphragm part 20. Like the diaphragm part
20, the diaphragm part 20A is formed in, for example, a Si
substrate. Thus, a silicon substrate or the like that is entirely
reduced in thickness to the thickness of a diaphragm may be used as
the diaphragm part 20A.
[0077] The glass substrate 30A has a monolithic structure and
includes a flat plate part 31 and a frame part 32. The frame part
32 is annularly formed on an outer edge portion of the flat plate
part 31 to vertically extend from the flat plate part 31. The glass
substrate 30A is a support member that supports the diaphragm part
20A. For example, an outer edge portion of a bottom surface of the
diaphragm part 20A is fixed to an upper surface of the frame part
32, which is a first surface 30Aa (a top surface in FIG. 9) of the
glass substrate 30A, by anodic bonding. The diaphragm part 20A is
bonded to the first surface 30Aa of the glass substrate 30A, so
that the cavity part 23, which is a hermetically sealed space, is
formed.
[0078] The function membrane 40 is formed so as to cover a bottom
surface of the flat plate part 31, which is a second surface 30Ab
(a bottom surface on the side opposite to the first surface 30Aa in
FIG. 9) of the glass substrate 30A. The details of the function
membrane 40 are as described above in the first embodiment.
[0079] Thus, in the first variation of the first embodiment as
well, the function membrane 40 is formed so as to cover the second
surface 30Ab. Therefore, the same effects as in the first
embodiment are produced.
[Second Variation of First Embodiment]
[0080] In a second variation of the first embodiment, another
physical quantity detection device having a structure different
from that of the first embodiment is illustrated. In the second
variation of the first embodiment, a description of the same
elements as those of the embodiment and variation described above
is omitted.
[0081] FIG. 10 is a cross-sectional view of a physical quantity
detection device according to the second variation of the first
embodiment. Referring to FIG. 10, a physical quantity detection
device 10B according to the second variation of the first
embodiment is different from the physical quantity detection device
10A according to the first variation of the first embodiment (FIG.
9) in that the diaphragm part 20A is replaced with a diaphragm part
20B. The physical quantity detection device 10B may be provided in,
for example, absolute pressure sensors.
[0082] Unlike the diaphragm part 20A (FIG. 9) of the physical
quantity detection device 10A, the diaphragm part 20B (a
pressure-sensitive diaphragm part), which is a physical quantity
detection part, has a frame-shaped projecting part 24 provided on
an outer edge portion of the flat plate so as to project in a
direction away from the glass substrate 30A. The diaphragm part 20B
operates the same as the diaphragm part 20A. Like the diaphragm
part 20A, the diaphragm part 20B is formed in, for example, a Si
substrate. Thus, a silicon substrate or the like that is partly
(that is, in a part other than the projecting part 24) reduced in
thickness to the thickness of a diaphragm may be used as the
diaphragm part 20B.
[0083] Thus, in the second variation of the first embodiment as
well, the function membrane 40 is formed so as to cover the second
surface 30Ab. Therefore, the same effects as in the first
embodiment are produced.
[Third Variation of First Embodiment]
[0084] In a third variation of the first embodiment, yet another
physical quantity detection device having a structure different
from that of the first embodiment is illustrated. In the third
variation of the first embodiment, a description of the same
elements as those of the embodiment and variations described above
is omitted.
[0085] FIGS. 11A and 11B are diagrams illustrating a physical
quantity detection device according to the third variation of the
first embodiment. FIGS. 11A and 11B are a plan view and a
cross-sectional view, respectively, of a physical quantity
detection device according to the third variation of the first
embodiment. In FIG. 11A, however, a below-described substrate 50
(including a pad 511 and a piezoresistive element 521) alone is
illustrated.
[0086] Referring to FIGS. 11A and 11B, a physical quantity
detection device 10C according to the third variation of the first
embodiment includes glass substrates 30B and 30C, function
membranes 40B and 40C, and the substrate 50. The physical quantity
detection device 100 may be provided in, for example, acceleration
sensors.
[0087] The substrate 50 includes a frame part 51, a beam part 52,
and a weight part 53. The beam part 52 supports the weight part 53.
One end of the beam part 52 is connected to the frame part 51. The
weight part 53 is formed at another end of the beam part 52. The
beam part 52 and the weight part 53 serve as a physical quantity
detection part, and are configured to be rotatable in directions
indicated by a double-headed arrow A in FIG. 11B (substantially
vertical directions) relative to the frame part 51. For example, a
Si substrate may be used as the substrate 50. In this case, the
frame part 51, the beam part 52, and the weight part 53 may be
formed of silicon as a monolithic structure.
[0088] A first surface 30Ba (a bottom surface in FIG. 11B) of the
glass substrate 30B including a cavity part 33 is bonded to an
upper surface of the frame part 51 of the substrate 50.
Furthermore, a first surface 30Ca (a top surface in FIG. 11B) of
the glass substrate 30C including a cavity part 34 is bonded to a
lower surface of the frame part 51 of the substrate 50. When the
substrate 50 is made of silicon, the substrate 50 and the glass
substrates 30B and 30C may be fixed by, for example, anodic
bonding. The cavity parts 33 and 34 communicate with each other to
form a hermetically sealed space. The beam part 52 and the weight
part 53, which serve as a physical quantity detection part, are
disposed in the hermetically sealed space.
[0089] A cavity space, however, may be formed in either the glass
substrate 30B or the glass substrate 30C alone. In that case, the
beam part 52 and the weight part 53 may be formed at such a
position where a gap is formed between the glass substrates 30B and
30C.
[0090] The piezoresistive element 521 is formed on the beam part
52. The weight part 53 is caused to rotate in either direction
indicated by the arrow A (substantially vertical direction) by the
application of acceleration, and the beam part 52 supporting the
weight part 53 also is caused to deflect upward or downward by the
movement of the weight part 53. The resistance value of the
piezoresistive element 521 on the beam part 52 is caused to change
by the deflection of the beam part 52, and the acceleration is
detected by detecting this change in the resistance value.
[0091] The pad 511, which is formed of aluminum or the like, is
formed outside the glass substrate 30B on the upper surface of the
frame part 51. The pad 511 is electrically connected to the
piezoresistive element 521 by diffusion wiring (not illustrated) or
the like. By connecting the pad 511 with an external integrated
circuit (IC) or the like, an acceleration sensor using the physical
quantity detection device 100 may be achieved.
[0092] The function membrane 40B is formed so as to cover a second
surface 30Bb (a top surface on the side opposite to the first
surface 30Ba in FIG. 11B) of the glass substrate 30B. The function
membrane 40C is formed so as to cover a second surface 30Cb (a
bottom surface on the side opposite to the first surface 30Ca in
FIG. 11B) of the glass substrate 30C. The function and material of
the function membranes 40B and 40C are the same as those of the
function membrane 40 illustrated in the first embodiment, and their
description is therefore omitted.
[0093] Thus, in the third variation of the first embodiment as
well, the function membranes 40B and 40C are formed so as to cover
the second surfaces 30Bb and 30Cb of the glass substrates 30B and
30C, respectively. Therefore, the same effects as in the first
embodiment are produced.
Second Embodiment
[0094] In a second embodiment, a physical quantity detector (a
semiconductor sensor) including the physical quantity detection
device 10 according to the first embodiment is illustrated. In the
second embodiment, a description of the same elements as those of
the embodiment described above is omitted. In the second
embodiment, the physical quantity detection device 10 may be
replaced with the above-described physical quantity detection
device 10A, 10B or 10C.
[0095] FIG. 12 is a cross-sectional view of a physical quantity
detector according to the second embodiment. Referring to FIG. 12,
a physical quantity detector 100 according to the second embodiment
includes the physical quantity detection device 10, a substrate
400, adhesive resin 500, a substrate 600, bonding wires 700a and
700b, and a lid 800.
[0096] To be more specific, the physical quantity detector 100 has
the following structure. That is, the substrate 600 is stepped to
include three-level surfaces. The substrate 400 is bonded onto a
lower-level surface 600a of the substrate 600 by the adhesive resin
500. A control IC may be mounted on the substrate 400.
[0097] The physical quantity detection device 10 is provided over
the substrate 400 with a resist spacer 310 interposed between the
physical quantity detection device 10 and the substrate 400. The
substrate 400 and the physical quantity detection device 10 are
bonded by adhesive resin 320 filling in a space around the resist
spacer 310 between the substrate 400 and the physical quantity
detection device 10.
[0098] The resist spacer 310 is formed by patterning a resist and
serves as a base for placing the physical quantity detection device
10. Furthermore, the resist spacer 310 serves to prevent
deformation of the adhesive resin 320 when a pressure is applied at
the time of bonding the bonding wires 700. The thickness of the
resist spacer 310 may be, for example, approximately 20 .mu.m to
approximately 30 .mu.m.
[0099] Pads 214 are provided on the diaphragm surface 21 of the
physical quantity detection device 10, and pads (not illustrated)
are also provided on the substrate 400 as wiring terminals. The
pads 214 of the physical quantity detection device 10 and the pads
(not illustrated) of the substrate 400 are electrically connected
by the bonding wires 700a.
[0100] Furthermore, pads (not illustrated) are also provided on a
middle-level surface 600b of the substrate 600 as wiring terminals.
The pads of the substrate 400 and the pads of the substrate 600 are
electrically connected by the bonding wires 700b. The lid 800 is
provided on an upper-level surface 600c of the substrate 600 so as
to cover the physical quantity detection device 10. Furthermore, a
through hole 810 is provided in the center of the lid 800 so as to
allow the diaphragm surface 21 to sense an external pressure.
[0101] The physical quantity detection device 10 is a device for
detecting a predetermined physical quantity, and detects an
absolute pressure in the physical quantity detector 100. Here, the
absolute pressure is a pressure relative to a perfect vacuum (or an
absolute vacuum). Therefore, the cavity part 23 of the physical
quantity detection device 10 is made a vacuum reference chamber
maintained in a vacuum state.
[0102] Thus, the physical quantity detector 100 that detects an
absolute pressure may be achieved using the physical quantity
detection device 10. Furthermore, the physical quantity detection
device 10 may be used for, in addition to semiconductor sensors
that detect an absolute pressure, gauge pressure sensors, flow
sensors, acceleration sensors, gyroscope sensors, laser
oscillators, optical switches, displays, optical sensors, prober
ring heads, IR sensors, .mu.-TAS (Micro Total Analysis Systems),
inkjet heads, micro motors, RF switches, etc.
[0103] All examples and conditional language provided herein are
intended for pedagogical purposes of aiding the reader in
understanding the invention and the concepts contributed by the
inventors to further the art, and are not to be construed as
limitations to such specifically recited examples and conditions,
nor does the organization of such examples in the specification
relate to a showing of the superiority or inferiority of the
invention. Although one or more embodiments of the present
invention have been described in detail, it should be understood
that the various changes, substitutions, and alterations could be
made hereto without departing from the spirit and scope of the
invention.
* * * * *